Electrostatic coating apparatus

ABSTRACT

A method and apparatus employs the currents of a high voltage electrostatic system to determine incipient grounded conditions. The electric current in the ground return to its high voltage electrostatic system is sensed to provide a signal. All alternating current components of the signal above substantially pure direct current are attenuated to provide a resulting DC signal that increases as a grounded article approaches a charged electrode. In one embodiment the resultant DC signal is sampled at a rate fast enough to acticipate the fastest expected approach of a grounded article to the charged electrode. Every other sample is retained so that it may be compared with the sample immediately following it. The samples are compared in a summing circuit which provides a signal corresponding to the change in conditions in the ground return circuit. A level is selected corresponding to incipient grounding of the charged high voltage electrode. If a difference of the successive samples exceeds this level, an alarm is actuated providing a useful output.

6 United States Patent 3,851,618 Bentley Dec. 3, 1974 ELECTROSTATICCOATING APPARATUS [75] Inventor: Stanley L. Bentley ABSTRACT [73]Assignee: Ransburg Corporation, Indianapolis, A method and apparatus p ythe Currents of a {mi high voltage electrostatic system to determineincipient grounded conditions. The electric current in the [22] Flled:1974 ground return to its high voltage electrostatic system is [21 1 N433,2 sensed to provide a signal. All alternating current components ofthe signal above substantially pure direct current are attenuated toprovide a resulting DC [52] US. Cl 118/7, 118/10, 118/11, Signal thatincreases as a grounded article approaches 8/626, 317/27 340/255 acharged electrode. In one embodiment the resultant [51] It'll. Cl B051)5/02 DC Signal i sampled at a rate fast enough to actici [58] F leld ofSearch 118/4, 48-495, pate the fastest expected approach of a groundedarm 1 18/627 626; 1 17/DIG' 2 cle to the charged electrode. Every othersample is retained so that it may be compared with the sample im- [56]References C'ted mediately following it. The samples are compared in aUNITED STATES PATENTS summing circuit which provides a signalcorrespond- 3,157,535 11/1964 Radke 118/7 ng o h h g in on itions in theground return 3,235,480 2/1966 Swartz et al. 1l8/49.1 X circuit. A levelis selected corresponding to incipient 3,347,701 /1967 Yamagishi et al.ll8/49.1 X grounding of the charged high voltage electrode. If a PrimaryExaminer-Morris Kaplan Attorney, Agent, or FirmMerrill N. Johnson; DavidH. Badger difference of the successive samples exceeds this level, analarm is actuated providing a useful output.

4 Claims, 4 Drawing Figures ARTICLE 13 I00 10 ll MOVEMENT GROUND HIGHCHARGED GROUNDED J RETURN VOLTAGE ELECTRODE ARTICLE NETWORK sOuRcE I 13oTIME |3b 14b LOW PASS OPERATIONAL NETWORK 2 |5b CLOCK CIRCUIT I I- lg-v|8\ GATED SAMPLING l9 CIRCUIT INVERTING GATED CIRCUIT 9b SAMPLING [7bCIRCUIT d 7 HOLDING I OIRcuIT 20b suMMINO 2 O CIRCUIT 2) LEvEL DETECTINGGil Try PATENTLL 3 851 .61 8

SHEET 10F 4 ARTICLE I3 I00 IO I Ix MOVEMENT GROUND HIGH CHARGED GROUNDEDRETURN VOLTAGE ELECTRODE ARTIOLE NETWORK SOURCE ah TIME 13b LOw PASSOPERATIONAL NETWORK |5b CLOCK CIRCUIT fi/ I8 I5cI GATED SAMPLING I9CIRCUIT INVERTING CIRCUIT Si'I IL L ING CIRCUIT 9 l7 HOLDING cIRcuITi200 I'm SUMMING 20/ CIRCUIT 2 b F/g. i 200 .L

LEVEL DETECTING (RV 2|cI PATENTE. 31974 SHEEF 2 OF 4 I l I PDnFDOPATENTEL 31974 SHEH 30F 4 ICLE I? I I0 llx EMENT GROUND HIGH CHARGED'GROUNDED URN VOLT ELECTRODE ARTICLE 1 WORK sou l3o my 0 70 OPERATI EPHASE NETWO SHIFT NETWORK K73 ,SUMMING' J Cl RCUIT 5 AMPLIFIER 1ELECTROSTATIC COATING APPARATUS This invention relates to an apparatusfor determining when a sharp-edged electrode charged to high voltage inan electrostatic system is being approached by a grounded object in sucha manner that dielectric breakdown of the atmosphere surrounding thehigh voltage electrode may occur with a resultant disruptive dischargeof electric energy stored on the high voltage electrode to the groundedobject. Common industrial systems include electrostatic coating systemswhich use high voltage electrodes to charge and deposit coating materialon an article. This invention particularly relates to a method andapparatus to be employed in such systems.

In industrial systems using electrostatics, potentials employed may beas high as 100,000 volts or more. These systems include high voltagesources to provide such potentials. Customarily, one terminal of thehigh voltage source is electrically grounded or connected to earth atzero potential. A high voltage electrode is commonly connected to theother potential of the high voltage source. ln most such industrialsystems, the high voltage electrode has a portion with a very smallradius of curvature which can be referred to as sharp. in most suchsystems, the sharp portion of the high voltage electrode is used toperform some operation. Electrostatic coating systems use high voltagein many different ways to accomplish depositon of coating materials.Several of these ways are disclosed in US. Pat. Nos. 2,893,893;2,893,894; Re. 24,602; 3,048,498; 3,l69,882 and 3,169,883.

The term electrosatic field is used to designate the condition in spaceexisting in the region occupied by two spaced electrodes. Anelectrostatic field, in a sense, depicts the manner in which the energystored in the high voltage electrostatic system is distributedthroughout the region between the high voltage electrode and thegrounded electrode or electrodes. Where the high voltage electrode issharp, the energy of the electrostatic system is concentrated adjacentto the sharp edge of the high voltage electrode. This concentration ofenergy is generally described by stating that the electrostatic fieldhas a high intensity in this region. The intensity of the electrostaticfield is measured by its potential gradient to indicate how the energyof the field is distributed throughout the region between theelectrodes, generally in such terms as kilovolts per centimeter.

Gases (including air which is a composite of many gases) may be ionizedby the energy of an intense electrostatic field. If, for example,molecules of the gases forming air are exposed to an intenseelectrostatic field where the local potential gradient is about or above30 kilovolts per centimeter, electrons are torn from the molecules underthe action of the intense field and the remaining portions of themolecules are thus ionized (that is, they have a net charge). Theionized gases move in response to the energy of the electrostatic fieldand may be used as a charging agent for coating material, for example.

One problem, however, is common to high voltage electrostatic systems.Because of the intense electrostatic fields that are used, ionization ofthe air can proceed under some conditions, to a complete disruption ofthe space between the high voltage electrode and the grounded electrode.This disruptive electrical disrent charge takes a form of a spark.Although local potential gradients on the order of 30 kilovolts percentimeter can exist closely adjacent to the sharp portion of anelectrode, when the average potential gradient throughout the spacebetween such a high voltage electrode and a grounded electrode exceedson the order of 12 kilovolts per centimeter, danger of disruptivedischarge is great. Such disruptive discharge generally carries most ofthe energy stored in the high voltage electrode to the grounded object.Since this energy can be great, such disruptive discharges can bedangerous.

In the electrostatic coating systems, past attempts to prevent sparkingfrom the high voltage electrodes in use have included attempts todevelop a signal from the electrical current flowing from the highvoltage to ground. These attempts have included the use of apparatussuch as that disclosed in early US. Pat. Nos. 2,509,277 and 2,650,329and later patents based upon this principle, such as US. Pat. No.3,641,971. Other patens for electrostatic coating have attempted tocontrol the voltage applied to the high voltage electrodes in responseto the current flowing from that electrode. Suchpatents are US. Pat. No.2,742,600 and US. Pat. No. 3,767,359.

These approaches to the solution of this problem in electrostaticcoating have not been completely successful, even though they have putto use improvements in circuit design and components which have beendeveloped since their innovation. Basically,-the reason for theirinability to provide satisfactory performance was their inability todetermine the incipient grounding condition from the electric currentflowing in the electrical systems of many industiral plants. Suchelectric currents include substantial components due to the otherinfluences, such as a substantial 60-cycle component and higherharmonics. 1

These higher harmonics include components due to the very high chargingcurrents flowing into the filter capacitors of the high voltage sourceas the output voltage of the secondary of the high voltage transformerexceeds the capacitor filter voltage. Other influences in the highvoltage system during its operation include the influence of linetransients created by other industrial equipment on the inputs of thehigh voltage transformer, the high frequency currents due to ionizationfrom the high voltage electrode and other less identifiable transientinfluences. Most, if not all, of these current components are many timesgreater than the curcomponent representing the approaching groundedarticle.

For example, the signal representing an approaching grounded object is avery small increase in the DC leakage current from the sharp highvoltage electrode, on the order of a few microamperes; however, theaverage DC load current on such systems can be as high as severalmilliamperes, and the AC current components can be even higher. Becauseof the variable effect of these transients, users of systems thatemployed the protective electrostatic coating apparatus of the typedisclosed in the patents identified above would set their apparatus sothat it would not sporadically initiate a signal in response to suchtransients. Their systems would not therefore detect incipient sparkingin many practical industrial installations.

This invention provides an apparatus employing the currents of a highvoltage electrostatic system to deterval), a signal is produced which isproportional to the turn current to the high voltage source and isprovided with high gain DC amplification and heavy attenuation of allfrequencies above several cycles per second. The signal is split intotwo channels that are summed to provide a determination of incipientgrounding.

This operation can be accomplished by a low pass operational networkincluding an active filter synthesized from operational amplifiers andproviding a transfer function l Vo/Vi --I-IWo /S aWoS W A clock isdesigned to provide time intervals to create a plurality of sampledsignals. The duration of the time intervals is determined by theresponse time needed to determine that the grounding condition isapproaching the high voltage electrode in the system. Two samplingcircuits are provided which are gated by signals from the clock. One ofthe sampling circuits is designed to retain the signal from the filterthat it receives during the interval that it isgated and to retain thissignal throughout the succeeding interval. The other sampling circuit isgated during the succeeding clock interval that is, the two samplingcircuits are gated alternately. The signal that occurs during the secondinterval is inverted and compared with the signal being retained fromthe first interval. This comparison of the signals from successive clockintervals provides a signal level which represents an increase ordecrease of the DC current from the high voltage electrode. This signalis fed to a level detector which is variable and can be set tocompensate for the varying DC load current conditions of eachinstallation.

When this method and apparatus is used, for example, in an electrostaticcoating system, it will react to a grounded part, such as an article tobe coated that is swinging on the conveyor, or a misaligned part beingcarried towards the high voltage electrode on the conveyor, withincreasing direct current from the high voltage electrode to theapproaching grounded part. This DC signal is sensed in the ground returnleadof the high voltage pack. All AC components in the ground returncurrent are rejected while the DC signal is amplified; thus, at theoutput of the low pass operational network, the signal is a DC levelwhich rises either at the rate of the approaching grounded object, or atthe response time of the filter, whichever is slower. The signal issampled at a rate rapid enough to react to the approaching groundedcondition, for example, 120 times per second. Comparison of thesuccessive signal samples is used to determine the approaching groundedcondition. Other transients are rejected by this method and apparatuswhich thus provides a reliable determination of an incipient groundingof the high voltage electrostatic system.

One further feature is built into this system. The retentive samplingcircuit has built into it a subtractive signal proportionate to the peaksignal level. Thus, when the signal being held (except for thesubtractive signal) in this sampling circuit is compared with the signalfrom the next successive gate interval (time interpeak magnitude of theDC signal during the first time 7 interval. Thus, this method andapparatus can determine when the DC current level in the ground returnis high enough to indicate the danger of sparking, irregardless ofwhether it is stable, increasing or decreasmg.

This operation can also be accomplished by another operational networkthat removes all components of the signal above pure DC. Thisoperational network in cludes a circuit synthesized from operationalamplifiers and providing an overall transfer function where K is thegain of the summer. In the operational network, the signal is split intotwo channels. In one channel the signal is operated upon by the transferfunction -ST/ 1 TS. This. network provides gains to only AC componentsof the signal. In the other channel the signal is only provided with aphase shift corresponding to the phase shift imposed on the signal bythe operational network in the first channel. The two signals are thencombined to result in the cancellation of the signal components aboveessentially pure DC. The resulting DC signal is then amplified and fedto a level detector which is variable and can be set to compensate forthe varying DC load conditions of these installations.

FIG. 1 is a block diagram showing the major components of one apparatusof this invention, and its method of operation for determining incipientgrounding of the high voltage electrostatic system.

FIG. 2 is a diagrammatic illustration showing the major circuit elementswithin the functional blocks of FIG. 1.

FIG. 3 is a block diagram showing the major components of anotherapparatus of this invention.

FIG. 4 is a diagrammatic illustration showing the major circuitcomponents within the functional blocks of FIG. 3.

FIG. 1 shows a typical high voltage electrostatic system in blockdiagram form. This system includes high voltage source 10, a chargedhigh voltage electrode 11, and grounded article 12. In electrostaticcoating systems, the grounded article 12 is free to move in thedirection of the charged electrode 11, shown by arrow 12a. Movement ofthe grounded article 12 in the direction of the charged electrode 11will result in an increased flow of electrical current across the spacebe-. tween the charged electrode and the grounded article. This electriccurrent will continue to increase until the space between the chargedelectrode and the grounded article breaks down and a spark is formed.

In order to determine the incipient grounding of the high voltageelectrostatic system, a ground return network 13 is connected betweenground and the ground terminal 10a of the high voltage source 10.Current flowing from the charged electrode to the grounded article willbe returned to the high voltage source through ground and the groundreturn network 13, and a signal will be developed by the ground returnnetwork at its output 13a. The signal generated in the ground returnnetwork is complex, being comprised of a plurality of DC and ACcomponents as illustrated in 13b. The signal includes substantial -cycleportions including many higher harmonics. This complex signal is appliedto a low pass operational network 14. The low pass operational networkis comprised of the high gain DC amplifier connected to transform theinput signal 13b with to the approach of the grounded article 12 to the7 charged electrode 11, signal at the output 14a of the low passoperational network 14 is a signal which increases in value as theobject approaches as shown at 14b. The increasing DC signal at theoutput 14a of the low pass operational amplifier 14 is sampled toprovide two successive signals in different intervals of time forcomparison. The sampling rate is determined by a clock circuit 15 whichprovides a series of gating impulses in more-or-less squarewave form atits output 15a, as shown in 15b. The duration t of each gate pulse, asshown in 15b, is selected to provide successive samples which occurrapidly enough so that comparison of these signals will provide adetectable increase in signal level under the most rapidly changingconditions to be anticipated.

The output of the clock circuit is applied to two gated samplingcircuits to and 18. The first gated sampling circuit 16 is coupled to aholding circuit 17 and is gated by the clock circuit 15 during the firstsampling interval tl. The signal 14b from the low pass operationalnetwork 14 is passed by the gated sampling circuit 16 and retained inholding circuit 17. Thus, at the output of the holding circuit 17a, thesignal 17b rises until the end of gate interval t1 in response to signal1412. At the end of time interval t1, the gating signal is removed fromthe input of the first gated sampling circuit and is applied to thesecond gated sampling circuit 18 for the time interval 12. The signalMb, during time interval t2, is passed through the gated samplingcircuit 18 and inverted by inverting circuit 19. The signal 19bappearing at the output 19a of the second gating sampling circuit andinverted circuit is a negative pulse representing in amplitude thesignal 14b, which occurs during the interval 12..

The output signals 17b of the holding circuit and 19b of the invertingcircuit represent those portions of the signal 14b which occur in thesuccessive time intervals 11 and 12. These outputs are fed to a summingcircuit 20. The instantaneous total or difference 20b at each instant oftime of these signals 17b and 19b appear at the output of the summingcircuit 200. Signal 2012, corresponding to the output of the summingcircuit, is applied to a level-detecting circuit 21. When the level ofsignal 20b reaches a level indicating that sparking may occur as shownat 20c, level-detecting circuit 21 produces an alarm signal 21!; at itsoutput 21a. This alarm signal can be used to actuate an alarm ordisconnect the high voltage, or for any other such purpose.

The apparatus of FIG. 1 thus performs the method of operation of thisinvention. The electric current in the ground return to its high voltageelectrostatic system is sensed to provide a signal. All alternatingcurrent components of the signal above substanially pure direct currentare attenuated to provide a resulting DC signal that increases as agrounded article approaches a charged electrode. The resultant DC signalis sampled at a rate fast enough to anticipate the fastest expectedapproach of grounded article to the charged electrode. Every othersample is retained so that it may be compared with the sampleimmediately following it. The samples are compared in a summing circuitwhich provides a signal corresponding to the change in conditions in theground return circuit. A level is selected corresponding to incipientgrounding of the charged high voltage electrode. If a difference of thesuccessive samples exceeds this level, an alarm is actuated providing auseful output.

FIG. 2 shows the apparatus of FIG. 1 in greater detail. The elements ofthe ground return network are shown within the dashed block 13 andinclude between ground and the ground terminal 10a of the high voltagesource 10, a fixed and a variable resistor, 22 and 23 respectively. Thevariable resistor 23 is used to adjust the level of the signal which isgenerated at output 13a of the ground return network. The ground returnnetwork also includes a fuse 24 and a current limiting resistor 25 whichfunctions with the input of the low pass operational network in themanner to be described.

The elements making up the low pass operational network include a zenerdiode 26. This zener diode, operating in conjunction with thecurrent-limiting resistor 25 and the other components of the groundreturn network, prevents the signal applied at the input of the low passoperational network from exceeding a preselected voltage level, forexample i 12 volts. The low pass operational network uses an operationalamplifier to transform the input signal in accordance with the transferfunction set forth above. By operational amplifier, I mean, for example,a Signetic Corporation N5741V type operational amplifier.

An active filter can be synthesized with the characteristics set forthabove using operational amplifiers and design techniques known to thoseskilled in the art as, for example, taught in Operational AmplifiersDesign & Application, Burr-Brown Research Corp. 1971, Library ofCongress No. 74-163297. Such an active filter having the desiredcharacteristics and using one operational amplifier 27 is shown in FIG.2. The plurality of resistances and capacitances connected at the outputand the input of the operational amplifier and numbered 28, 29, 30, 31and 32 generate from the terminal 26a to terminal 33 the desiredtransfer function. In this case, the signal at the output of theoperational amplifier 33 is without any significant AC component. Thesignal at terminal 33 is a substantially total DC signal which will riseand fall as a grounded object approaches, or retreats from, the chargedhigh voltage electrode of the electrostatic system. The typical valuesof such components which would provide the characteristics set forthabove are resistor 28-1 1.3 kohms, capacitor 29-2.2 microfarads,resistor 31-1 13 kohms, capacitor 32-0.! microfarads, resistor 30-102kohms. The output of the active filter is amplified by operationalamplifier 34 and resistances 3S and 36 are presented at the output 14aof the low pass operational network 14.

Sampling of the signal at the output 14a of the low pass operationalnetwork 14 is provided by the clock circuit shown within the dashedlines representing blocks 15. The clock circuit includes a transformer37. The primary of the transformer 38a is connected to a 60-cycle powersource. The secondary 38b is connected to ground at one side; the otherside is coupled to an operational amplifier 39 through a resistor 40.Feedback from the output of the operational amplifier 39 to its input iseffected by two diodes 41 and 42. This circuit is connected to -12 voltsthrough variable resistor 43. Diodes 41 and 42 and operational amplifier39 form in the connection shown in FIG. 2 a to volt limiter circuit. Thepositive half-cycle of the AC line is omitted and the negativehalf-cycle is clipped at 5 volts, inverted by the operational amplifier,and formed into a square pulse to yield a 0 to +5 volt pulse of about 8milliseconds.

The output of operational amplifier 39 is connected through the resistor44 to transistor 45 which is driven into saturation by this signal onalternative half-cycles of the 60-cycle input.

When transistor 45 is not driven into saturation by the gate signal ofthe clock, transistor 46, by the action of resistors 47 and 48, is insaturation, effectively removing collector voltage from transistor 49 inthe second gated sampling circuit within the dashed block 18. Duringthis half-cycle of 60-cycle input to the clock (which corresponds to thetime interval t1 as shown at b of FIG. 1), collector voltage is appliedto transistor 50 of the first gated sampling circuit within the dashedblock 16. The signal output'of the low pass operational network 14,during this time interval t1, is applied to transistor 50 throughresistor 51. This signal is amplified by transistor 50 and is integratedand stored by the holding circuit within dashed a resistor 52 andcapacitor 54. A diode 53, inserted between the emitter of transistor 50and the holding circuit l7 protects the transistor 50. The resistor 52connected across capacitor 54' provides the appartus with the ability todetect relatively stationary conditions corresponding to incipientgrounding of the high voltage charged electrode in a manner that will beexplained below.

In the alternate half-cycle of the 60-cycle input (corresponding to t2at 15b of FIG. 1), transistor 45 is in saturation, removing collectorvoltage from transistor 50; however, transistor 46 is not in saturationand collector voltage is applied to transistor 49 of the second gatedsampling circuit 18. During interval 12, the signal from the output 140of the low pass operational amplifier 14 is applied through resistor 55in the second gated sampling circuit and protective diode 56 to aninverter circuit connected with transistor 49. The signal is transmittedto the inverting circuit within the dashed box 19, comprised of anoperational amplifier 57, connected with resistors 58, 59 to perform theinversionamplification function.

The signals at the outputs of the holding circuit 17a and of theinverting circuit 19a are fed to a summing circuit within the dashed box20. The output of the holding circuit 17 passes through a variableresistor 60 to the operational amplifier 61. The output of the invertingcircuit 19 is sent to the operational amplifier through resistor 62. Afeedback resistor 63, in conjunction with the variable resistor 60,resistor 62 and the operational amplifier 61, sums and provides the netdifference of the retained signal from interval t1 which appears at theoutput 170 of the holding circuit 17 and the signal during the intervalt2 which appears at the ouput 19a of the inverter circuit 19. Thisdifference appears at the output a of the summing circuit 20, but isinverted by operation of the circuit.

block 17 which includes Level-detecting circuit within the dashed lines21 detects when the difierence between the signals which occur duringinterval t1 and which occur during t2 exceed a predetermined selectedlevel that corresponds to incipient grounding of the high voltageelectrostatic system. The level-detecting system may be of any kindsufficient to reliably detect a signal level. As shown in FIG. 2,level-detector circuit 21 includes zener diode 64 to hold the input tothe monostable multivibrator 67 within predetermined voltage limits, forexample i: 5 volts. The monostable multivibrator 67, connected withresistors 65 and capacitors 66, will trigger if the level appliedexceeds a predetermined preset value.

To provide an apparatus which will detect slowly moving or stationaryconditions, suddenly imposed and representing incipient grounding of thehigh voltage electrostatic system, resistor 52 is connected acrosscapacitor 54 in the holding circuit 17. During the interval t2, resistor52 bleeds the stored charge from capacitor 54 that represents theintegrated signals occurring during that period :1. The charge bledthrough the resistor reduces, during time interval t2, the level of thesignal at the output of the holding circuit 17a to be summed with theinverted signal appearing at the output 19a of the inverter circuit, andresults in an increased difference at the output of the summing circuit20a. Thus, if incipient grounding of the high voltage electrostaticelectrode is suddenly imposed by a very rapidly moving grounded objectin a period less than the sampling interval (which in the deviceillustrated in FIG. 2 is oneone hundred and twentieth of a second) thatsubsequently remains relatively stationary, a large difference signalwill nevertheless be generated by the apparatus andactuate thelevel-detecting circuit to provide an ,alarm.

FIG. 3 shows a high voltage electrostatic system using anotherembodiment of this that of FIG. 1, includes a high voltage source 10, acharged high voltage electrode 11 and a grounded article 12. Thegrounded article 12 is free to move in the direction of the chargedelectrode 11 as shown by arrow 12a. As indicated above, movement of thegrounded article 12 in the direction of charged electrode 11 will resultin a spark unless the incipient grounding condition is detected.

Like the embodiment shown in FIG. 1, a ground return network 13 isconnected between ground and the ground terminal 10a of the high voltagesource 10. Current flowing from the charged electrode to the groundedarticle will be returned to the high voltage source through the groundand ground return network 13, and a signal will be developed by theground return network at its output 130. As indicated before, thissignal is complex, being comprised of a plurality of DC and ACcomponents as illustrated in 13b of FIG. 1. The apparatus shown in FIG.3 operates on this complex signal to determine incipient groundingconditions.

The ground return signal at terminal 13a of the ground return network issplit at 70 into two channels. In the first channel an operationalnetwork 71 transforms the signal by the transfer function This transferfunction eliminates the DC signals from its output 71a and like allnetworks, provides a phase shift to the signal.

The second channel includes an operational phase shift network 72 whichpasses the entire signal at the invention. This system, like output ofthe ground return network with virtually the same amplification andphase shift as operational network 71 imposes upon the AC components ofthe signal.

As a result of the operation of operational network 71, the output at71a of the first channel in an amplified inversion of the AC componentspresent in the signal. At the output 72a of the second channel, theentire signal is present with the same amplification and delay of thefirst channel. The signals from the first and second channels are sentto a summing network 73 whose output at 73a is, therefore, essentially aDC signal. This signal is amplified by signal 74 whose output 74a isconnected to a level-detecting circuit 75 of any appropriate type.

The apparatus of FIG. 3 thus performs the method ofoperation of thisinvention. The electric current in the ground return to its high voltageelectrostatic system is sensed to provide a signal. All alternatingcurrent components of the signal above substantially DC are attenuatedto provide a resulting DC signal that increases as the grounded articleapproaches the charged electrode. This signal is generated by splittingthe signal into two channels, one of which amplifies and inverts onlythe AC components of the signal, which are then summed with the signalin the other channel which uniformly amplifies the entire signal to thesame degree as the first channel. The signals are summed to provide asignal corresponding to the change in conditions in the ground returncircuit. A level is selected corresponding to incipient grounding of thecharged high voltage electrode and providing a useful output if theamplified signal exceeds this level.

FIG. 4 shows the apparatus of FIG. 3 in greater detail. The elements ofthe ground return network are shown within the dashed block 13 and aredescribed above. The elements making up the operational network isdashed block 71 are synthesized to provide the transfer function ST/l+STusing operational amplifiers and the design techniques as, for example,taught in Operational Amplifiers Design and Application" (identifiedabove), using one operational amplifier 76. Zener diodes 70a and 70b areconnected to terminal 70 to prevent the signal at tenninal 70 fromexceeding preselected levels. The plurality and resistances andcapacitances 77, 78, 79, and 81 generate between terminal 70 andterminal 71a the desired transfer function. This operational networkattenuates the DC components of the complex signal 13b and segregatesits AC components.

The signal at terminal 70 is also applied to the phase shift networkshown within dashed block 72, including an operational amplifier 82 andresistors 83 and 84. The function of this phase shift network is toprovide amplification of the entire signal at terminal 70 and a phaseshift corresponding to that imposed on the signal by operational network71. The outputs of operational networks 71 and phase shift network 72are summed by a summing circuit shown within the dashed outline 73 whichincludes an operational amplifier 85 and resistors 86 and 87. Resistors80 and 87 are to provide temperature stability and do not contribute tothe overall transfer function of the circuit. The output at 73a is asignal that is substantially pure DC. The AC components of the signalhave been cancelled by virtue of the operations of operational networks71 and 72 and the summing circuit 73. The resultant DC signal is relatedto the approach of a grounded article to the high voltage electrode.

This signal is amplified by the amplifier in dashed block 74 and fed toa level-detecting circuit which can be set to be actuated at a signallevel corresponding to incipient grounding conditions.

Though I have shown specific circuits in describing the methods andapparatus of my invention, it will be apparent that other specificcircuits may be .devised to practice the invention.

I claim:

1. In an electrostatic coating system including a high voltage supplyhaving two terminals;

a conveyor for transporting articles to be coated;

an electrostatic coating device connected with the one terminal of thehigh voltage supply to create electrostatic deposition of atomizedcoating material;

and means connected between the other terminal of the high voltagesupply and ground to sense the electric current between the high voltagesupply and ground, and to disconnect the high voltage supply from itsenergizing source, the improvement in which the means connected betweenthe above terminal and ground includes a low pass type operationalnetwork to provide a signal with attenuation of all electrical currentcomponents significantly above direct current, while amplifying directcurrents;

a clock to generate a sampling gate for the signal;

two gated sampling circuits to detect the direct current level of thesignal when gated by the clock, one of the gated sampling circuits beingconnected to a holding circuit for the detected signal for one cycle ofthe gating signal, and the other gated sampling circuit being connectedto an inverting circuit to invert the signal from the other gatedsampling circuit;

a summer to determine the difference of the signal from the holdingcircuit and the inverted signal from the other gated sampling circuit;and

a level detector to generate an operative signal in the event the levelexceeds a predetermined level which represents incipient grounding ofthe high voltage electrode.

2. A system of claim I, wherein the low pass operational networkincludes an active filter designed to transform the voltage input intoan output by the transfer function HWO2/S2+OKWOS+WO2 in which H equals10, W0 equals 10 cycles per second, and :1 equals 0 2 for maximally flatButterworth Response with 40 db/decade roll-off.

3. A system of claim 2 wherein the clock circuit provides gating pulseswith a time interval of th of a second to apply to the gated samplingcircuits.

4. A system of claim 1 wherein the first holding circuit includes meansto subtract a signal from its output proportional to the magnitude ofthe output signal during the time interval generated by the clock.

*' t k II

1. In an electrostatic coating system including a high voltage supplyhaving two terminals; a conveyor for transporting articles to be coated;an electrostatic coating device connected with the one terminal of thehigh voltage supply to create electrostatic deposition of atomizedcoating material; and means connected between the other terminal of thehigh voltage supply and ground to sense the electric current between thehigh voltage supply and ground, and to disconnect the high voltagesupply from its energizing source, the improvement in which the meansconnected between the above terminal and ground includes a low pass typeoperational network to provide a signal with attenuation of allelectrical current components significantly above direct current, whileamplifying direct currents; a clock to generate a sampling gate for thesignal; two gated sampling circuits to detect the direct current levelof the signal when gated by the clock, one of the gated samplingcircuits being connected to a holding circuit for the detected signalfor one cycle of the gating signal, and the other gated sampling circuitbeing connected to an inverting circuit to invert the signal from theother gated sampling circuit; a summer to determine the difference ofthe signal from the holding circuit and the inverted signal from theother gated sampling circuit; and a level detector to generate anoperative signal in the event the level exceeds a predetermined levelwhich represents incipient grounding of the high voltage electrode.
 2. Asystem of claim 1, wherein the low pass operational network includes anactive filter designed to transform the voltaGe input into an output bythe transfer function -HWo2/S2+ Alpha WoS+Wo2 in which H equals 10, Woequals 10 cycles per second, and Alpha equals Square Root 2 formaximally flat Butterworth Response with 40 db/decade roll-off.
 3. Asystem of claim 2 wherein the clock circuit provides gating pulses witha time interval of 120th of a second to apply to the gated samplingcircuits.
 4. A system of claim 1 wherein the first holding circuitincludes means to subtract a signal from its output proportional to themagnitude of the output signal during the time interval generated by theclock.